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  1. JDK
  2. JDK-8295673

Deprecate and disable legacy parallel class loading workaround for non-parallel-capable class loaders

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    • b23

    Description

      double_lock_wait was added in JDK 6 to workaround the custom class loader deadlock problem [1] before parallel capable class loaders were added.

      It's expected that custom class loaders using non-hierarchical class delegation model should migrate to parallel-capable class loaders to avoid the deadlock problem as described in [1].

      This double_lock_wait logic should be removed.

      [1] https://openjdk.org/groups/core-libs/ClassLoaderProposal.html

      double_lock_wait does the following:

      // We only get here if this thread finds that another thread
      // has already claimed the placeholder token for the current operation,
      // but that other thread either never owned or gave up the
      // object lock
      // Waits on SystemDictionary_lock to indicate placeholder table updated
      // On return, caller must recheck placeholder table state
      //
      // We only get here if
      // 1) custom classLoader, i.e. not bootstrap classloader
      // 2) custom classLoader has broken the class loader objectLock
      // so another thread got here in parallel
      //
      // lockObject must be held.
      // Complicated dance due to lock ordering:
      // Must first release the classloader object lock to
      // allow initial definer to complete the class definition
      // and to avoid deadlock
      // Reclaim classloader lock object with same original recursion count
      // Must release SystemDictionary_lock after notify, since
      // class loader lock must be claimed before SystemDictionary_lock
      // to prevent deadlocks
      //
      // The notify allows applications that did an untimed wait() on
      // the classloader object lock to not hang.

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              coleenp Coleen Phillimore
              mchung Mandy Chung
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